Light emitting element and method of making same

ABSTRACT

A light emitting element has a substrate of gallium oxides and a pn-junction formed on the substrate. The substrate is of gallium oxides represented by: (Al X In Y Ga (1−X−Y) ) 2 O 3  where 0≦x≦1, 0≦y≦1 and 0≦x+y≦1. The pn-junction has first conductivity type substrate, and GaN system compound semiconductor thin film of second conductivity type opposite to the first conductivity type.

[0001] This application is based on Japanese patent application Nos.2003-137912 and 2002-160630, the entire contents of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates to a light emitting element with a widebandgap enough to emit visible light to ultraviolet light and a methodof making the same, and relates particularly to a light emitting elementemploying a colorless, transparent and conductive substrate thattransmits emit visible light to ultraviolet light, offering a verticalstructure in electrode configuration, and allowing emitted light to beoutputted from the substrate side and a method of making the same.

[0004] 2. Description of the Related Art

[0005] Conventionally, a light emitting element with a composition ofSiC substrate/n-GaN/p-GaN is known (e.g., Japanese patent applicationlaid-open No. 2002-255692).

[0006] SiC is brown and transparent material, and it transmits visiblelight up to about 427 nm. Therefore, a light emitting element employingthe SiC substrate allows emitted light to be outputted from thesubstrate side.

[0007] The light emitting element employing a SiC substrate ismanufactured by epitaxially growing SiC thin film on a SiC singlecrystal wafer to get the SiC substrate, then growing n-GaN and p-GaNlayers on the substrate, cutting out light emitting element chips.

[0008] However, there is a serious problem in the light emitting elementemploying the SiC substrate. The SiC single crystal wafer has a badcrystalline quality such that there exist micro-pipe defects penetratingvertically in the SiC single crystal wafer. Therefore, it is requiredthat, when making chips from a wafer having n-GaN and p-GaN layers grownthereon, the wafer must be carefully cut while avoiding the micro-pipedefect. This causes a bad efficiency in the manufacture of lightemitting element.

[0009] On the other hand, SiC transmits up to light of blue region butdoes not transmit light of ultraviolet region. When GaN-emitted lightincluding visible light to ultraviolet light is outputted from thesubstrate side, the light of ultraviolet region cannot be transmittedtherethrough. Thus, ultraviolet light cannot be outputted from thesubstrate side. Furthermore, SiC is brown-colored and, therefore, whentransmitting light through SiC, part of emitted light wavelength must beabsorbed.

SUMMARY OF THE INVENTION

[0010] It is an object of the invention to provide a light emittingelement that employs a colorless, transparent and conductive substratethat transmits emit visible light to ultraviolet light, offers avertical structure in electrode configuration, and allows emitted lightto be outputted from the substrate side and a method of making the same.

[0011] It is another object of the invention to provide a light emittingelement with a good manufacturing efficiency and a method of making thesame.

[0012] According to the invention, a light emitting element comprises:

[0013] a substrate of gallium oxides; and

[0014] a pn-junction formed on the substrate.

[0015] According to another aspect of the invention, a light emittingelement comprises:

[0016] a single crystal substrate of oxides including gallium as themajor component; and

[0017] compound semiconductor thin film formed on the single crystalsubstrate.

[0018] According to a further aspect of the invention, a method ofmaking a light emitting element, comprises the steps of:

[0019] growing a single crystal substrate including gallium as the majorcomponent by EFG(Edge-defined film Fed Growth) method where, in ahigh-temperature vessel of a controlled atmosphere, using a slit diethat allows source material melt to be continually lifted above the slitdie through the capillary phenomenon of a slit provided with the slitdie and a crucible that accommodates the slit die and the sourcematerial melt, single crystal the cross section of which has the sameshape as the top surface of the slit die is grown; and

[0020] growing compound semiconductor thin film on the substrate.

[0021] According to a further aspect of the invention, a method ofmaking a light emitting element, comprises the steps of:

[0022] providing single crystalline Ga₂O₃ system seed crystal andnon-single crystalline Ga₂O₃ system material;

[0023] growing a single crystal substrate including gallium as the majorcomponent by FZ(Floating Zone) method where the Ga₂O₃ system seedcrystal and Ga₂O₃ system material are contacted and heated such that theGa₂O₃ system seed crystal and Ga₂O₃ system material are melted at thecontacting portion, thereby crystallize the Ga₂O₃ system material; and

[0024] growing compound semiconductor thin film on the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Preferred embodiments of the invention will be explained withreference to the drawings, wherein:

[0026]FIG. 1 is a graph showing a temperature dependency of resistivityof β-Ga₂O₃;

[0027]FIG. 2 is a partly cross sectional and perspective view showing acrucible 6 to be inserted into FEG pulling vessel used in a method ofmaking a light emitting element according to the invention;

[0028]FIG. 3 is a partly cross sectional view showing an infraredheating single-crystal growing apparatus used in a method of making alight emitting element according to the invention;

[0029]FIG. 4 shows an atom arrangement in the case that GaN (001) facethin film is grown on (101) face of β-Ga₂O₃ system single crystalsubstrate;

[0030]FIG. 5 shows an atom arrangement in the case that GaN (001) facethin film is grown on (001) face of Al₂O₃ system crystal substrate;

[0031]FIG. 6 is an illustration showing an MOCVD apparatus used in amethod of making a light emitting element according to the invention;

[0032]FIG. 7 is a cross sectional view showing a first example of lightemitting element according to the invention;

[0033]FIG. 8 is a cross sectional view showing a modification of thefirst example in FIG. 7;

[0034]FIG. 9 is a cross sectional view showing a second example of lightemitting element according to the invention;

[0035]FIG. 10 is a cross sectional view showing a third example of lightemitting element according to the invention; and

[0036]FIG. 11 is a cross sectional view showing a fourth example oflight emitting element according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0037] [Substrate]

[0038] β-Ga₂O₃ substrate is conductive and, therefore, a vertical-typeLED in electrode configuration can be made. As a result, the entirelight emitting element of the invention forms a current flowing pathand, thereby, the current density can be lowered and the life of thelight emitting element can be elongated.

[0039]FIG. 1 shows resistivity measurements of β-Ga₂O₃ substrate.Measuring the resistivity of β-Ga₂O₃ substrate with n-type conductivity,as shown in FIG. 1, a resistivity of about 0.1 Ωcm is obtained at roomtemperature. Furthermore, the temperature dependency of resistivity issmall in the temperature range where the light emitting element will beexactly used. Therefore, the light emitting element using the β-Ga₂O₃substrate has an excellent stability.

[0040] Due to offering the vertical-type LED in electrode configuration,the step of etching its n-layer to expose to form an n-electrode thereonis not needed. Therefore, the number of manufacturing steps can bedecreased and the number of chips obtained per unit area of substratecan be increased. The manufacturing cost could be lowered.

[0041] In contrast, when a sapphire substrate is used, the electrodeconfiguration must be horizontal. In this case, after thin layers ofIII-V compound semiconductor such as GaN is grown, the step of etchingand masking the n-layer to expose to form an n-electrode thereon isadditionally needed to install the n-electrode. Comparing with this,when the electrode configuration is vertical as the case of GaAs systemlight emitting elements, the steps of etching and masking the n-layer isnot needed.

[0042] When a SiC substrate is used, the lattice mismatch between SiCand GaN is substantially large. In case of SiC, multiple phases of 3C,4H, 6H, 15R etc. exist and, therefore, it is difficult to obtain asubstrate in a single phase. Also, due to the very high hardness, theprocessability is not good and it is difficult to get a smoothsubstrate. Observing it in atom scale, there exist a lot of steps withdifferent phases on the surface of substrate. When a thin layer is grownon such a Sic substrate, it must have multiple crystalline types anddifferent defect densities. Namely, in growing the thin layer on the SiCsubstrate, a number of cores with different crystalline qualities arefirst grown on the substrate and then the thin layer is grown such thatthe cores are combined. Therefore, it is extremely difficult to improvethe crystalline quality of the thin layer. The lattice mismatch betweenSiC and GaN is reported theoretically 3.4%, but it is, in fact,considerably greater than that value due to the above reasons.

[0043] In comparison with SiC, β-Ga₂O₃ is of a single phase and has asmooth surface in atom scale. Therefore, β-Ga₂O₃ does not have such asubstantially large lattice mismatch as observed in SiC. From theviewpoint of bandgap, SiC, e.g., 6H-SiC has a bandgap of 3.03 eV and isnot transparent in the wavelength range of shorter than about 427 nm.Considering that the entire light emission range of III-V systemcompound semiconductors is about 550 to 380 nm, the available wavelengthrange of SiC is only about two thirds of the entire range. In contrast,β-Ga₂O₃ is transparent in the range of longer than about 260 nm, whichcovers the entire light emission range of III-V system compoundsemiconductors, and is available particularly in ultraviolet region.

[0044] The substrate applicable to the invention, though it is basicallyof β-Ga₂O₃, may be of an oxide that includes Ga(gallium) as the majorcomponent and, as the minor component, at least one selected from thegroup of Cu, Ag, Zn, Cd, Al, In, Si, Ge and Sn. The minor componentfunctions to control the lattice constant or bandgap energy. Forexample, the substrate may be (Al_(X)In_(Y)Ga_((1−X−Y)))₂O₃, where0≦x≦1, 0≦y≦1, and 0≦x+≦1.

[0045] [Thermal Expansion Coefficient]

[0046] From the viewpoint of thermal expansion, comparing GaN having athermal expansion coefficient of 5.6×10⁻⁶/K, β-Ga₂O₃ has that of4.6×10⁻⁶/K, which is nearly equal to sapphire (4.5×10⁻⁶/K) and moreadvantageous than 6H-SiC (3.5×10⁻⁶/K). Thus, to reduce the difference ofthermal expansion coefficients therebetween is a key factor in enhancingthe quality of grown film.

[0047] [Bulk Single Crystal]

[0048] The most advantageous point of β-Ga₂O₃ is that it can give a bulksingle crystal. In the light emission region of near-infrared to redobtained from, typically, GaAs system material, a bulk single crystal isalways available and it allows a thin layer having an extremely smalllattice mismatch to the substrate to be grown on the conductivesubstrate. Therefore, it is easy to make a light emitting element withlow cost and high light emission efficiency.

[0049] In contrast, for GaN system and ZnSe system materials expected togive a blue LED, it is, in fact, impossible to give a bulk singlecrystal. In the filed of these material systems, it has been a greatdeal tried to make a bulk single crystal that is conductive andtransparent in the light emission region and that has a sufficientlysmall lattice mismatch. However, even now, this problem is not solved.The β-Ga₂O₃ substrate of the invention can perfectly solve the problem.The invention enables the manufacture of the bulk single crystal with adiameter of 2 inches by EFZ method or FZ method and, thereby, the blueto ultraviolet LEDs can be developed in the same way as GaAs system LED.

[0050] [Manufacture of Ga₂O₃ Single Crystal by EFG Method]

[0051]FIG. 2 shows a crucible that is used to manufacture a Ga₂O₃ singlecrystal by EFG method. The crucible 6 is used being inserted to an EFGpulling vessel (not shown). The crucible 6 is of, e.g., iridium and isprovided with a slit die 8 having a slit 8 a through the capillaryphenomenon of which β-Ga₂O₃ melt 9 is lifted.

[0052] The growth of single crystal by EFG method is performed asfollows. β-Ga₂O₃ as raw material is entered a predetermined amount intothe crucible 6, being heated to melt, and, thereby, β-Ga₂O₃ melt 9 isobtained. β-Ga₂O₃ melt 9 is lifted above the slit die 8 provided in thecrucible 6 through the capillary phenomenon of the slit 8 a to contact aseed crystal 7. Being cooled, a grown crystal 10 having an arbitrarysectional form is obtained.

[0053] In detail, the crucible 6 of iridium has an inner diameter of48.5 mm, a thickness of 1.5 mm, and a height of 50 mm. Into the crucible6, Ga₂O₃ of 75 g as raw material is entered. Then, the slit die 8 whichis 3 mm thick, 20 mm wide, 40 mm high and 0.5 mm slit interval is set inthe crucible 6. The crucible 6 is kept 1,760° C. in ordinary atmosphereof nitrogen and at oxygen partial pressure of 5×10⁻² under 1 atm, theseed crystal 7 of β-Ga₂O₃ is contacted to the β-Ga₂O₃ melt 9 beinglifted through the capillary phenomenon of the slit 8 a. The growthspeed of single crystal is 1 mm/h.

[0054] The single crystal is grown on the slit die 8 while being definedby the shape of the slit die 8 and, therefore, the thermal gradient atthe crystal growing interface is considerably smaller than CZ method.Further, β-Ga₂O₃ melt 9 is supplied through the slit 8 a and the crystalgrowth speed is higher than the diffusion speed of β-Ga₂O₃ melt 9 in theslit 8 a. Therefore, the evaporation of components included in β-Ga₂O₃melt 9, i.e., a variation in composition of β-Ga₂O₃ melt 9 can beeffectively suppressed. As a result, a high-quality single crystal canbe obtained. Also, to increase the size of single crystal can be easilyachieved by increasing the slit die 8 because the shape of grown crystal10 is defined by the shape of the slit die 8. Thus, EFG method offersthe increased size and high quality of Ga₂O₃ single crystal, which werehard to achieve by CZ method.

[0055] [Manufacture of Ga₂O₃ Single Crystal by FZ Method]

[0056]FIG. 3 shows an infrared heating single-crystal growing apparatusthat is used to manufacture a Ga₂O₃ single crystal by FZ(Floating Zone)method. The infrared heating single-crystal growing apparatus 100includes: a silica tube 102; a seed rotation section 103 that holds androtates a seed crystal 107 of β-Ga₂O₃ (hereinafter referred to as “seedcrystal 107”); a raw-material rotation section 104 that holds androtates a polycrystalline raw material 109 of β-Ga₂O₃ (hereinafterreferred to as “polycrystalline raw material 109”); a heater 105 thatheats the polycrystalline raw material 109 to melt it; and a controller106 that controls the seed rotation section 103, raw-material rotationsection 104 and the heater 105.

[0057] The seed rotation section 103 includes: a seed chack 133 thatholds the seed crystal 107; a lower rotating shaft 132 that rotates theseed chack 133; and a lower drive section that drives the lower rotatingshaft 132 to rotate clockwise and move upward and downward.

[0058] The raw-material rotation section 104 includes: a raw-materialchack 143 that holds the top end 109 a of polycrystalline raw material109; an upper rotating shaft 142 that rotates the raw-material chack143; and an upper drive section 141 that drives the upper rotating shaft142 to rotate counterclockwise and move upward and downward.

[0059] The heater 105 includes: a halogen lamp 151 that heats thepolycrystalline raw material 109 from the diameter direction to melt it;an elliptic mirror 152 that accommodates the halogen lamp 151 andconverges light radiated from the halogen lamp 151 to a predeterminedportion of the polycrystalline raw material 109; and a power source 153that supplies power to the halogen lamp 151.

[0060] The silica tube 102 accommodates the lower rotating shaft 132,the seed chack 133, the upper rotating shaft 142, the polycrystallineraw material 109, a single crystal 108 of β-Ga₂O₃ and the seed crystal107. The silica tube 102 is structured such that mixed gas of oxygen gasand nitrogen gas as inert gas can be supplied and sealed therein.

[0061] The growth of β-Ga₂O₃ single crystal is performed as follows.First of all, the seed crystal 107 and polycrystalline raw material 109are prepared. The seed crystal 107 is obtained, e.g., by cutting outβ-Ga₂O₃ along the cleaved surface. It has a diameter of less than onefifth of that of grown crystal or a sectional area of less than 5 mm²and has such a strength that it does not break when growing the β-Ga₂O₃single crystal. The polycrystalline raw material 109 is obtained bycharging a predetermined amount of Ga₂O₃ powder into a rubber tube(notshown), cold-compressing it at 500 MPa, then sintering it at 1500° C.for ten hours.

[0062] Then, the end of the seed crystal 107 is fixed to the seed chack133, and the top end 109 a of polycrystalline raw material 109 is fixedto the raw-material chack 143. The top end of the seed crystal 107 iscontacted to the bottom end of the polycrystalline raw material 109 bycontrolling upward and downward the position of the upper rotating shaft142. The positions of upper rotating shaft 142 and the lower rotatingshaft 132 are controlled upward and downward such that light of thehalogen lamp 151 is converged on the top end of the seed crystal 107 andthe bottom end of the polycrystalline raw material 109. The atmosphere102 a in the silica tube 102 is filled with mixed gas of nitrogen andoxygen, the composition of which may vary between 100% nitrogen and 100%oxygen, at a total pressure of 1 to 2 atm.

[0063] When an operator turns on a power switch(not shown), thecontroller 106 starts the single crystal growth control to therespective sections according to a control program as follows. Turningon the heater 105, the halogen lamp 151 starts heating the top end ofthe seed crystal 107 and the bottom end of the polycrystalline rawmaterial 109, melting their contacting portions to make a melt drop. Inthis stage, only the seed crystal 107 is kept rotating.

[0064] Then, the seed crystal 107 and the polycrystalline raw material109 are rotated in the opposite direction to each other such that theircontacting portions are melted while being sufficiently mixed together.When a suitable amount of single crystal melt 108′ of β-Ga₂O₃ isobtained, the polycrystalline raw material 109 stops rotating and onlythe seed crystal 107 keeps rotating, and then the polycrystalline rawmaterial 109 and the seed crystal 107 are pulled in the oppositedirection to each other, i.e., upward and downward respectively to forma dash-neck which is thinner than the seed crystal 107.

[0065] Then, the seed crystal 107 and the polycrystalline raw material109 are heated by the halogen lamp 151 while being rotating at 20 rpm inthe opposite direction to each other, and the polycrystalline rawmaterial 109 is pulled upward at a rate of 5 mm/h by the upper rotatingshaft 142. In heating the polycrystalline raw material 109, thepolycrystalline raw material 109 is melt to form the single crystal melt108′, which is then cooled to grow β-Ga₂O₃ single crystal 108 that has adiameter of the same as or less than the polycrystalline raw material109. When a suitable length of single crystal is grown, the β-Ga₂O₃single crystal 108 is extracted.

[0066] Next, the manufacture of substrate using β-Ga₂O₃ single crystal108 is conducted as follows. β-Ga₂O₃ single crystal 108 has a strongcleavage at (100) face when it is grown in b-axis <010> orientation andis, therefore, cleaved at planes parallel and vertical to (100) face toget a substrate. Alternatively, when it is grown in a-axis <100>orientation or c-axis <001> orientation, the cleavage at (100) or (001)face becomes weak and, therefore, the processability at all planesbecomes good and there is no limitation about the cleaved surface.

[0067] Advantages in the manufacture of β-Ga₂O₃ single crystal 108 by FZmethod are as follows.

[0068] (1) Large sized β-Ga₂O₃ single crystal 108 having a diameter ofmore than 1 cm can be obtained since it is grown in a predetermineddirection.

[0069] (2) High crystalline quality can be obtained while suppressingthe cracking and eutectic crystallization when β-Ga₂O₃ single crystal108 is grown in a-axis <100>, b-axis <010> or c-axis <001> orientation.

[0070] (3) β-Ga₂O₃ single crystal 108 can be produced in goodreproducibility and can be, therefore, applied to as a substrate forvarious semiconductor devices.

[0071] [Growth of II-VI Group Compound ZnSe Thin Film on β-Ga₂O₃Substrate]

[0072] On (101) face of β-Ga₂O₃ system single crystal substrate, ZnSethin film with p-type conductivity is grown at 350° C. by MOCVD (metalorganic chemical vapor deposition). Source gases for ZnSe are Dimethylzinc and H2Se. Nitrogen as p-dopant is supplied by preparing NH3atmosphere. Nitrogen is, as acceptor, doped while being substituted forSe. II group element may be Zn, Cd and Hg, and IV group element may beO, S, Se, Te and Po. II-VI system compound applicable to the inventionis, for example, ZnSe, ZnO etc.

[0073] [Growth of III-V Group Compound Thin Film on β-Ga₂O₃ Substrate]

[0074] III-V group compound thin film is grown by MOCVD. III groupelement may be B, Al, Ga, In and Tl, and V group element may be N, P,As, Sb and Bi. III-V system compound applicable to the invention is, forexample, GaN, GaAs etc.

[0075]FIG. 4 shows an atomic arrangement in the case that GaN thin filmis grown on (101) face of β-Ga₂O₃ system single crystal substrate. Inthis case, (001) face of GaN is grown on (100) face of β-Ga₂O₃ systemsingle crystal. O atoms 70, 70, . . . are arranged on (101) face ofβ-Ga₂O₃ system single crystal. In FIG. 4, oxygen (O) atoms 70 arerepresented by solid-line circle. The lattice constant of β-Ga₂O₃ systemsingle crystal in (101) face is a=b=0.289 nm, γ≈116°. The latticeconstant of GaN in (001) face is a_(G)=b_(G)=0.319 nm, γ_(G)=120°. InFIG. 4, nitrogen (N) atoms 80 are represented by dot-line circle.

[0076] When GaN thin film is formed such that (001) face of GaN is grownon (101) face of β-Ga₂O₃ system single crystal, the mismatch of latticeconstant is about 10%, and the mismatch of angle is about 3%. Thus, thearrangement of oxygen atoms of β-Ga₂O₃ system single crystal is nearlyidentical to that of nitrogen atoms of GaN, and, therefore, GaN thinfilm can have a uniform plane structure. Even when GaN thin film isformed on (101) face of β-Ga₂O₃ system single crystal without a bufferlayer to be inserted therebetween, there is no problem about latticemismatch.

[0077] Alternatively, when In is added to β-Ga₂O₃ single crystal toadjust the lattice constant, the lattice constant of GaN in (001) facebecomes closer to that of β-Ga₂O₃ system single crystal in (101) face.Therefore, GaN thin film can have a more uniform plane structure.

[0078]FIG. 5 shows an atomic arrangement in the case that GaN thin filmis grown on Al₂O₃ system crystal substrate. O atoms 75, 75, . . . arearranged on (001) face of Al₂O₃ system crystal. In FIG. 5, oxygen (O)atoms 75 are represented by solid-line circle. The lattice constant ofAl₂O₃ system crystal in (001) face is a_(A)=b_(A)=0.475 nm, γ_(A)=120°.The lattice constant of GaN in (001) face is a_(G)=b_(G)=0.319 nm,γ_(G)=120°. In FIG. 5, nitrogen (N) atoms 80 are represented by dot-linecircle.

[0079] When GaN thin film is formed such that (001) face of GaN is grownon (001) face of Al₂O₃ system crystal, the mismatch of lattice constantis about 30%. Therefore, when GaN thin film is formed on Al₂O₃ systemcrystal without a buffer layer to be inserted therebetween, there occursa problem that due to the lattice mismatch therebetween GaN thin filmmay have no uniform plane structure.

[0080] [Method of Growing Thin Film on β-Ga₂O₃ Substrate]

[0081]FIG. 6 schematically shows a MOCVD apparatus. The MOCVD apparatus20 includes: a reaction vessel 21 equipped with an exhaust system 26that includes a vacuum pump and an exhauster (not shown); a susceptor 22on which a substrate 27 is mounted; a heater 23 to heat the susceptor22; a control shaft that rotates and moves upward and downward thesusceptor 22; a silica nozzle 25 that supplies source gases in theoblique or horizontal direction to the substrate; a TMG gas generator 31that generates trimethyl gallium (TMG) gas; a TMA gas generator 32 thatgenerates trimethyl aluminum (TMA) gas; and a TMI gas generator 33 thatgenerates trimethyl indium (TMI) gas. If necessary, the number of gasgenerators may be increased or decreased. Nitrogen source is NH₃ andcarrier gas is H₂. When growing GaN thin film, TMG and NH₃ are used.When growing AlGaN thin film, TMA, TMG and NH₃ are used. When growingInGaN thin film, TMI, TMG and NH₃ are used.

[0082]FIG. 7 shows the cross sectional structure of a light emittingelement 40 including semiconductor layers grown by the MOCVD apparatus20 in FIG. 6.

[0083] The process of growing the semiconductor layers by the MOCVDapparatus 20 is as follows. First of all, the substrate 27 is mounted onthe susceptor 22 while facing up the film-forming surface, and then itis set into the reaction vessel. At a temperature of 1020° C., TMG of54×10⁻⁶ mol/min, NH₃ of 4 l/min, H₂ of 2 l/min and monosilane (SiH₄) of2×10⁻¹¹ mol/min are flown for 60 min, thereby 3 μm thick Si-doped GaN(n-GaN) layer 1 a is grown.

[0084] Then, at a temperature of 1030° C., TMG of 54×10⁻⁶ mol/min, NH₃of 4 l/min, H₂ of 2 l/min and biscyclopentadienyl magnesium (CP₂Mg) of36×10⁻⁶ mol/min are flown for 20 min, thereby 1 μm thick Mg-doped GaN(p-GaN) layer 1 b is grown. On the layer 1 b, transparent electrode(Au/Ni) 1 h is deposited and then Mg-doped GaN 1 b is made p-type byannealing. A bonding electrode (p-electrode) 1 c is formed on thetransparent electrode 1 h, and a bonding wire 1 f is bonded to thebonding electrode 1 c while forming a ball 1 e. Then, an n-electrode 1 dis formed on the bottom surface of the substrate 27. Thus, the lightemitting element 40 is obtained.

[0085] The electrodes are formed by deposition or sputtering etc. Theyare of materials that offer ohmic contact to the layer or substrate onwhich the electrodes are formed. For example, to n-type conductivitylayer or substrate, any one of metals including Au, Al, Co, Ge, Ti, Sn,In, Ni, Pt, W, Mo, Cr, Cu, and Pb, or an alloy including two or more ofthe metals (e.g., Au—Ge alloy), or two-layer structure selected from themetals (e.g., Al/Ti, Au/Ni and Au/Co), or ITO (indium tin oxide) isused. To p-type conductivity layer or substrate, any one of metalsincluding Au, Al, Be, Ni, Pt, In, Sn, Cr, Ti and Zn, or an alloyincluding two or more of the metals (e.g., Au—Zn alloy and Au—Be alloy),or two-layer structure selected from the metals (e.g., Ni/Au), or ITO isused.

[0086] [Forming of Thin Layers with Different Carrier Concentrations]

[0087] For example, on n-GaN layer, another n-GaN layer with a carrierconcentration lower than the n-GaN layer is formed, and, on the lowercarrier concentration n-GaN layer, p-GaN layer and another p-GaN layerwith a carrier concentration higher than the p-GaN layer are formed inthat order. The carrier concentration can be differentiated by changingthe amount of n-dopant or p-dopant added.

[0088] When using the substrate of β-Ga₂O₃ system single crystal andforming, on the substrate, aplurality of n-type layers with differentcarrier concentrations and a plurality of p-type layers with differentcarrier concentrations, advantages (1) to (4) stated below are obtained.

[0089] (1) Due to a carrier concentration lower than the substrate, then-GaN layer grown on the substrate has a good crystalline quality andthereby the light emission efficiency is enhanced.

[0090] (2) Due to the junction of n-GaN layer and p-GaN layer, the lightemitting element having pn-junction is formed and therefore the lightemission of a short wavelength is obtained through the bandgap of GaN.

[0091] (3) Due to being of β-Ga₂O₃ system single crystal, the substratecan enjoy a high crystalline quality and n-type good conductivity.

[0092] (4) Due to being of β-Ga₂O₃ system single crystal, the substratecan transmit light of ultraviolet region and therefore ultraviolet lightto visible light can be emitted from the substrate side.

[0093] [Forming of Buffer Layer]

[0094]FIG. 8 shows a modification that the light emitting element inFIG. 7 further includes a buffer layer. Between β-Ga₂O₃ single crystalsubstrate 27 of the invention and n-GaN layer 1 a, there is formed aAl_(X)Ga_(1−X)N buffer layer (0≦x≦1). The buffer layer is grown by theabove-mentioned MOCVD apparatus. The p-n junction structure is grown bythe above-mentioned method of growing thin film on β-Ga₂O₃ substrate.

[0095] Examples of this invention are stated below.

EXAMPLE 1 Forming of n-GaN Thin Film on p-type Conductivity Substrate

[0096] The p-type conductivity substrate is made as follows. First,β-Ga₂O₃ single crystal is prepared by FZ method. The β-Ga₂O₃polycrystalline raw material is obtained by uniformly mixing, forexample, β-Ga₂O₃ including MgO (p-dopant source) and charging apredetermined amount of the mixture into a rubber tube, cold-compressingit at 500 MPa to form a stick, then sintering it at 1500° C. for tenhours in the atmosphere. Thereby, β-Ga₂O₃ system polycrystalline rawmaterial including Mg is obtained. By another way, β-Ga₂O₃ seed crystalis provided. Under the growth atmosphere with total pressure of 1 to 2atm, flowing mixture gas of N₂ and O₂ at 500 ml/min, the β-Ga₂O₃ seedcrystal and β-Ga₂O₃ system polycrystalline raw material are contacted toeach other in the silica tube, and they are heated such that the β-Ga₂O₃seed crystal and β-Ga₂O₃ system polycrystalline raw material are meltedat the contacting portion. The melting β-Ga₂O₃ seed crystal and β-Ga₂O₃system polycrystalline raw material are rotated at 20 rpm in theopposite direction to each other, and β-Ga₂O₃ single crystal is grown ata rate of 5 mm/h during the rotation. As a result, on the β-Ga₂O₃ seedcrystal, transparent and insulating β-Ga₂O₃ system single crystalincluding Mg is obtained. The β-Ga₂O₃ system single crystal is used asthe substrate. The substrate is then annealed at a predeterminedtemperature (e.g., 950° C.) in oxygen atmosphere for a period and,thereby, the number of oxygen defects is decreased to give the p-typeconductivity substrate.

[0097] Then, the n-type conductivity thin film is formed on thesubstrate obtained. The thin film is grown by MOCVD method. First, thep-type conductivity substrate is set into the MOCVD apparatus. Keepingthe substrate temperature at 1150° C., H₂ of 20 l/min, NH₃ of 10 l/min,TMG of 1.7×10⁻⁴ mol/min and monosilane (SiH₄) diluted to 0.86 ppm by H₂of 200 ml/min are flown for 30 min. Thereby, about 2.2 μm thick n-typeconductivity GaN thin film with a carrier concentration of 1.5×10¹⁸/cm³is formed.

EXAMPLE 2 Light Emitting Element with pn-junction

[0098]FIG. 9 shows a light emitting element with pn-junction mounted ona printed circuit board. The light emitting element 40 includes: a Ga₂O₃substrate 41 of β-Ga₂O₃ single crystal: a Al_(X)Ga_(1−X)N buffer layer42 (0≦x≦1) formed on the Ga₂O₃ substrate 41; a n-GaN layer 43 formed onthe Al_(X)Ga_(1−X)N buffer layer 42; a p-GaN layer 44 formed on then-GaN layer 43; a transparent electrode 45 formed on the p-GaN layer 44;a Au bonding electrode 47 formed on part of the transparent electrode45; and a n-electrode 46 formed on the bottom surface of the Ga₂O₃substrate 41. The light emitting element 40 is mounted on the printedcircuit board 50 through a metal paste 51 and a bonding wire 49 isbonded to the bonding electrode 47 while forming a ball 48.

[0099] The light emitting element 40 emits light at the pn-junctioninterface where the n-GaN layer 43 and p-GaN layer 44 are bonded.Emitted light is output such that part of emitted light is output, asoutput light 60, upward through the transparent electrode 45 and anotherpart is first directed to the bottom of the Ga₂O₃ substrate 41,transmitting through the substrate 41, then being outputted upward afterbeing reflected by the metal paste 51. Thus, the light emissionintensity of is increased comparing the case that emitted light isdirectly output.

EXAMPLE 3 Flip-Chip Type Light Emitting Element

[0100]FIG. 10 shows a flip-chip type light emitting element. The lightemitting element 40 includes: a Ga₃O₃ substrate 41 of β-Ga₂O₃ singlecrystal; a Al_(X)Ga_(1−X)N buffer layer 42 (0≦x≦1) formed on the Ga₂O₃substrate 41; a n-GaN layer 43 formed on the Al_(X)Ga_(1−X)N bufferlayer 42; a p-GaN layer 44 formed on part of the n-GaN layer 43; an-electrode 46 formed on the n-GaN layer 43; and a p-electrode 52 formedon the p-GaN layer 44. The light emitting element 40 is flip-chip bondedthrough solder balls 63, 64 beneath the p-electrode 52 and n-electrode46 to lead frames 65, 66.

[0101] The light emitting element 40 emits light at the pn-junctioninterface where the n-GaN layer 43 and p-GaN layer 44 are bonded.Emitted light is output, as output light 60, upward transmitting throughthe Ga₂O₃ substrate 41.

EXAMPLE 4 Double-Heterostructure Light Emitting Element

[0102]FIG. 11 shows a double-heterostructure light emitting element. Thelight emitting element 40 includes: a Ga₂O₃ substrate 41 of β-Ga₂O₃single crystal; a Al_(Y)Ga_(1−Y)N buffer layer 42 (0≦y≦1) formed on theGa₂O₃ substrate 41; a n-Al_(Z)Ga_(1−Z)N cladding layer 55 (0≦z≦1) formedon the Al_(Y)Ga_(1−Y)N buffer layer 42; a In_(m)Ga_(1−m)N light-emittinglayer 56 (0≦m≦1) formed on the n-Al_(Z)Ga_(1−Z)N cladding layer 55; ap-Al_(P)Ga_(1−P)N cladding layer 57 (0≦p<1,p>Z) formed on theIn_(m)Ga_(1−m)N light-emitting layer 56; a transparent electrode 45formed on the p-Al_(P)Ga_(1−P)N cladding layer 57; a Au bondingelectrode 47 formed on part of the transparent electrode 45; and an-electrode 46 formed on the bottom surface of the Ga₂O₃ substrate 41.The light emitting element 40 is mounted on the printed circuit board 50through a metal paste 51 and a bonding wire 49 is bonded to the bondingelectrode 47 while forming a ball 48.

[0103] The bandgap energy of the n-Al_(Z)Ga_(1−Z)N cladding layer 55 isgreater than that of the In_(m)Ga_(1−m)N light-emitting layer 56, andthe bandgap energy of the p-Al_(P)Ga_(1−P)N cladding layer 57 is greaterthan that of the In_(m)Ga_(1−m)N light-emitting layer 56.

[0104] The light emitting element 40 has the double-heterostructurewhere electron and hole as carriers are confined in the In_(m)Ga_(1−m)Nlight-emitting layer 56 to increase the probability of recombinationtherebetween. Therefore, the light emission efficiency can be remarkablyenhanced.

[0105] Furthermore, emitted light is output such that part of emittedlight is output, as output light 60, upward through the transparentelectrode 45 and another part is first directed to the bottom of theGa₂O₃ substrate 41, transmitting through the substrate 41, then beingoutputted upward after being reflected by the metal paste 51. Thus, thelight emission intensity of is increased comparing the case that emittedlight is directly output.

[0106] <Advantages of the Invention>

[0107] (1) According to the invention, a light emitting element and amethod of making the same that use the transparent and conductivesubstrate of bulk β-Ga₂O₃ single crystal can be provided. The lightemitting element can be equipped with electrodes formed on the top andbottom and, therefore, the structure can be simplified to enhance themanufacturing efficiency. Also, the efficiency of light outputted can beenhanced.

[0108] (2) Due to employing the β-Ga₂O₃ system material, the substratecan be colorless, transparent and conductive. It can transmit visiblelight to ultraviolet light. The light emitting element made by using thesubstrate can have the vertical structure. Also, emitted light can beoutputted from the substrate side.

[0109] (3) Furthermore, the β-Ga₂O₃ single crystal substrate has aprocessability better than a substrate of conventional materials, i.e.,sapphire and SiC.

[0110] Although the invention has been described with respect to thespecific embodiments for complete and clear disclosure, the appendedclaims are not to be thus limited but are to be construed as embodyingall modifications and alternative is constructions that may occur to oneskilled in the art which fairly fall within the basic teaching hereinset forth.

What is claimed is:
 1. A light emitting element, comprising: a substrateof gallium oxides; and a pn-junction formed on said substrate.
 2. Alight emitting element according to claim 1, wherein: said substrate isof gallium oxides represented by: (Al_(X)In_(Y)Ga_((1−X−Y)))₂O₃ where0≦x≦1, 0≦y≦1 and 0≦x+y≦1.
 3. A light emitting element according to claim1, wherein: said pn-junction includes first conductivity type saidsubstrate, and GaN system compound semiconductor thin film of secondconductivity type opposite to the first conductivity type.
 4. A lightemitting element according to claim 1, wherein: said pn-junctionincludes first conductivity type GaN system compound semiconductor thinfilm formed on first conductivity type said substrate, and GaN systemcompound semiconductor thin film of second conductivity type opposite tothe first conductivity type.
 5. A light emitting element according toclaim 4, wherein: said first conductivity type GaN system compoundsemiconductor thin film includes first conductivity type GaN systemcompound semiconductor thin film with a first predetermined bandgapenergy, and first conductivity type GaN system compound semiconductorthin film with a second predetermined bandgap energy smaller than saidfirst predetermined bandgap energy; and said second conductivity typeGaN system compound semiconductor thin film has a third predeterminedbandgap energy greater than said second predetermined bandgap energy. 6.A light emitting element according to claim 5, wherein: said firstconductivity type GaN system compound semiconductor thin film with thefirst predetermined bandgap energy is of one material selected fromIn_(a)Ga_(1−a)N, GaN and Al_(b)Ga_(1−b)N, where 0<a<1 and 0<b<1; saidfirst conductivity type GaN system compound semiconductor thin film withthe second predetermined bandgap energy is of In_(c)Ga_(1−c)N, where0<c<1 and a<c; and said second conductivity type GaN system compoundsemiconductor thin film with the third predetermined bandgap is of onematerial selected from GaN and Al_(d)Ga_(1−d)N, where 0<d<1.
 7. A lightemitting element, comprising: a single crystal substrate of oxidesincluding gallium as the major component; and compound semiconductorthin film formed on said single crystal substrate.
 8. A light emittingelement according to claim 7, wherein: said substrate is conductive andtransmits visible light and ultraviolet light.
 9. A light emittingelement according to claim 7, wherein: said compound semiconductor thinfilm is of III-V group compound.
 10. A light emitting element accordingto claim 7, wherein: said compound semiconductor thin film is of II-VIgroup compound.
 11. A light emitting element according to claim 7,wherein: said substrate has n-type conductivity; and said compoundsemiconductor thin film has p-type conductivity.
 12. A light emittingelement according to claim 7, wherein: said substrate has p-typeconductivity; and said compound semiconductor thin film has n-typeconductivity.
 13. A light emitting element according to claim 7,wherein: said substrate is of Ga₂O₃ system single crystal.
 14. A lightemitting element according to claim 7, wherein: said substrate is of(101) face Ga₂O₃ system single crystal; and said compound semiconductorthin film is of GaN formed on said (101) face Ga₂O₃ system singlecrystal.
 15. A light emitting element according to claim 7, wherein: atleast one of said substrate and said compound semiconductor thin filmincludes a component to adjust the lattice constant, the latticeconstant
 16. A light emitting element according to claim 15, wherein:said component is of one or more selected from the group of Cu, Ag, Zn,Cd, Al, In, Si, Ge and Sn.
 17. A light emitting element according toclaim 7, wherein: said compound semiconductor thin film includes one ormore n-type conductivity layer and one or more p-type conductivitylayer.
 18. A light emitting element according to claim 9, wherein: saidcompound semiconductor thin film includes V group element the atomarrangement of which is nearly identical to that of oxygen atom includedin said substrate.
 19. A method of making a light emitting element,comprising the steps of: growing a single crystal substrate includinggallium as the major component by EFG (Edge-defined film Fed Growth)method where, in a high-temperature vessel of a controlled atmosphere,using a slit die that allows source material melt to be continuallylifted above the slit die through the capillary phenomenon of a slitprovided with the slit die and a crucible that accommodates the slit dieand the source material melt, single crystal the cross section of whichhas the same shape as the top surface of the slit die is grown; andgrowing compound semiconductor thin film on said substrate.
 20. A methodof making a light emitting element, comprising the steps of: providingsingle crystalline Ga₂O₃ system seed crystal and non-single crystallineGa₂O₃ system material; growing a single crystal substrate includinggallium as the major component by FZ (Floating Zone) method where saidGa₂O₃ system seed crystal and Ga₂O₃ system material are contacted andheated such that said Ga₂O₃ system seed crystal and Ga₂O₃ systemmaterial are melted at the contacting portion, thereby crystallize saidGa₂O₃ system material; and growing compound semiconductor thin film onsaid substrate.